1. Field of the Invention
The present invention relates generally to an analog filter in a wireless transmission/reception device, and more particularly, to an analog filter for correcting a cut-off frequency in a wireless transmission/reception device, and a method for setting a cut-off frequency using the same.
2. Description of the Related Art
Generally, in wireless communication systems, reception devices use an analog filter to remove unwanted signals, such as, for example, noises from received baseband signals, and to obtain desired channel signals. The analog filter must set an exact cut-off frequency in order to obtain the desired channel signals. Specifically, the setting of the exact cut-off frequency in the analog filter may influence performance of wireless communication systems.
FIG. 1 is graph illustrating characteristics of a Low Pass Filter (LPF), which is an example of an analog filter. Specifically, FIG. 1 shows a relationship between a frequency value and a decibel (dB) value, indicating a gain value at a particular frequency, in describing a cut-off frequency of an LPF.
Generally, most of signals present in nature, such as, for example, sound and light waves, increase in strength exponentially. In order to more easily process signals with these properties, analog circuits express gain values and cut-off frequency values in a logarithmic scale.
For example, when a gain value is expressed in a logarithmic scale, the gain value undergoes logarithmic computation, is multiplied by 20, and is then used in dB. Additionally, when power is expressed in a logarithmic scale, the power value undergoes logarithmic computation, is multiplied by 10, and is then used in dB.
Common filters vary in input-output gain values as a frequency increases. Therefore, these filters have a pass band and a stop band on the entire frequency band. A boundary frequency, which is a reference frequency for distinguishing between the pass band and the stop band, is referred to as a ‘cut-off frequency fc’.
For example, in the LPF, the cut-off frequency fc is defined as a frequency having a gain value that is 3 dB lower than a Direct Current (DC) in the pass band, or a gain value at the low frequency. FIG. 1 shows that a gain value in the DC is Adc (dB) and a gain value at the cut-off frequency fc is Adc-3 (dB). Specifically, the gain value at the cut-off frequency fc is 3 dB lower than the gain value in DC.
FIG. 2 is a diagram illustrating an analog filter having the characteristic function of FIG. 1.
Referring to FIG. 2, an amplifier 150 may change its gain value and cut-off frequency fc by varying resistances of variable resistors 160 and 170. A gain value of the amplifier 150 in DC may be defined as
            R      b              R      c        ,and the cut-off frequency fc may be defined as
      1          2      ⁢      π      ⁢                          ⁢              R        b            ⁢      C        ,where Ra represents a resistance of an input variable resistor 160, Rb represents a resistance of a feedback variable resistor 170, and C represents a capacitance of a feedback capacitor 180.
However, since the resistances of the resistors 160 and 170 and capacitance of the capacitor 180, which constitute the analog circuit, may vary depending on the temperature and process conditions, their exact values may not be predicted. Therefore, even though a cut-off frequency fc is set in an analog filter, the set cut-off frequency fc may be different from its target value. Thus, in the analog filter having the structure of FIG. 2, the cut-off frequency fc is set by manually adjusting the variable resistor 170.
Commonly, an analog filter uses variable resisters whose resistances vary linearly. A cut-off frequency fc of the analog filter is inversely proportional to the resistances of the variable resistors. Therefore, in order to set an exact cut-off frequency fc, the resistance and capacitance should coincide with their designed values. However, the resistance and capacitance may deviate from their designed values by a maximum of 30% due to changes in temperature and manufacturing processes.
By manually compensating for the deviation by extracting and applying several samples, a deviation due to normal distribution during the manufacturing process may be accurately compensated for. Additionally, a deviation due to a change in a time-varying temperature may also be compensated for.
A digital modem in reception devices, constituting wireless communication systems, compensates for signals that are output from the analog filter and then quantized. Additionally, a Phase Compensation Filter (PCF) compensates for a group phase delay of the signals output from the analog filter at a digital stage following the quantization stage.
However, a range in which the digital modem may achieve compensation is limited, and when the cut-off frequency is not set exactly, the digital modem may not fully filter out the noise frequencies (or blockers) adjacent to the signal frequency. This inability to fully filter significantly decreases the Signal to Noise Ratio (SNR) in the reception devices causing a degradation of the call quality and an increase in power consumption.
Further, if the cut-off frequency used in the analog filter is deviated, a digital phase correction filter for compensating for the group phase delay may worsen the phase delay, causing a deterioration of reception performance.